Thirty-two countries were involved in World War I. The United States, Russia, Italy, Britain and France were the core of the Allied Forces, and they fought against a group of countries known as the Central Powers, which included Austria-Hungary, Bulgaria, Germany and the Ottoman Empire.
Many of the countries involved in the first World War had problems with each other before its start, but the war officially began when Austria-Hungary declared war on Serbia after a Serb national assassinated the country's leader. Germany declared war on Russia and France a short time later, and this pattern continued as more countries entered the fray. The United States took a neutral stance at the beginning, but after Germany sank several passenger ships that killed many Americans, it also entered the war.
Other major forces that eventually become involved in the first World War include China, Belgium, Italy, Greece, Japan, Montenegro, Liberia, Romania and Portugal. These countries were initially reluctant to go to war until the United States became involved. Despite some of them having strong ties with Central Power countries, most joined as part of the Allied Forces. Before the war ended, Guatamela, Nicaragua, San Marino, Siam, Panama and the Honduras also became involved in World War I.